1. Field of the Invention
This invention relates to passive vehicle classification and more specifically to a system and method for passively sensing a vehicle's low frequency electro-magnetic (EM) emanations and identifying harmonic interrelationships, strong non-harmonic signals, and temporal properties to classify the vehicle.
2. Description of the Related Art
Passive vehicle detection and classification systems rely on passively sensed visual, thermal, seismic and acoustical data to first detect and then classify vehicles. Technicians are trained to recognize patterns in the sensed data as particular vehicle characteristics and then use them to classify the vehicle. Depending upon the application, the type of vehicles involved, and the environment in which the detection takes place, it might not be practical to gather certain types of data. This reduces the total amount of data that is available to the technician to classify the vehicle. Furthermore, the sensed data may be very noisy due to sensor characteristics, environmental conditions or other sources in the area. Lastly, each type of data can characterize only certain aspects of the vehicle. As a result, the sensed data may not characterize the vehicle with sufficient specificity to classify it with the desired confidence and precision.
The SURTASS system developed by Hughes Electronics, the assignee of the present invention, uses one or more acoustic sensors to passively sense acoustic emanations from passing submarines. No other types of data are gathered and used in the classification process. The acoustic data is passed to a computer that filters the data and then computes its temporal frequency response or spectrogram, commonly known as a LOw Frequency Analysis Response (LOFARgram), which is displayed as a time-frequency-intensity plot on a terminal. The technician is trained to recognize distinctive acoustic patterns in the LOFARgram and classify the submarine.
The VLF Home Page found on the Internet at dlc@nov a.stanford.edu as updated on Jun. 13, 1995 presented a discussion of lightning-generated whistlers in the Earth's geomagnetic field. A network of low frequency (0-10 kHz) magnetic sensors are used to detect the magnetic emanations associated with lightning. These measurements are typically performed in remote locations in the Northern Hemisphere or Antarctica to avoid other sources of low frequency magnetic signals such as the strong 60 Hz signal associated with local power grids or the low frequency emanations generated by vehicles, which would appear as noise and tend to obscure the lightning. To observe and study the lightning-generated whistlers, the sensed magnetic signals are first mapped into audio signals that sound like decaying whistles and displayed in coordinates of frequency versus time, with the intensity indicated by the color of the display. This illustrates what lightning looks and sounds like in the low frequency spectrum.